Fixed default sprite sheet type used by -sheet in the CLI to "rows"

Older versions of Aseprite used the rows as a default type (not
horizontal), because if we specified -split-layers, each layer would
have its own row.

src/app/cli/cli_processor.cpp

@@ -568,9 +568,9 @@ void CliProcessor::process(Context* ctx)
     }
 
     if (m_exporter) {
-      // Horizontal sprite sheet as the default type
+      // Rows sprite sheet as the default type
       if (sheetType == SpriteSheetType::None)
-        sheetType = SpriteSheetType::Horizontal;
+        sheetType = SpriteSheetType::Rows;
       m_exporter->setSpriteSheetType(sheetType);
 
       m_delegate->exportFiles(ctx, *m_exporter.get());